This invention relates to devices for measuring radiation, and may be used for example in analyzing combustion processes, e.g., heat transfer in the combustion chambers of gas turbine engines.
The life and reliability of engine components, and in particular combustor walls, has always been a subject of a great deal of interest. The evaluation of combustor wall life is done by using various equations and empirical relations. A lot of this in turn depends on the temperatures of the walls. These temperatures are a consequence of various convective, conductive and radiative heat fluxes within the combustion gas. Therefore it is important both to be able to measure and to be able to model radiative heat transfer accurately.
The present modelling of radiative heat transfer requires input of absorption coefficients. An absorption coefficient is a function of wavelength, local gas temperature and gas composition, although the present modelling techniques use grey gas absorption coefficients (ones that do not depend on wavelength).
A known technique (the Schmidt method) for making radiation measurements in a combustion chamber uses a detector located outside a window in one wall of the combustion chamber and a background radiation source (a furnace) located in a line of sight outside a window in an opposite wall. A chopper is positioned in front of the radiation source to enable alternate measurements along the line of sight with the radiation source, and without it (i.e., measurement of just the emission from the combustion process). Since the radiation emitted by the furnace is known, this gives the proportion of radiation that is absorbed by the flame in the combustion chamber. This quantity is often known as absorbtivity of the flame. It is also possible to deduce radially weighted values of temperatures. The temperatures so determined are sometimes called Schmidt temperatures or brightness temperatures because they are not strictly temperatures. Norgen and Claus (NASA technical notes TN6410,1970; TMX-3394,1976; and technical paper 1722,1981) have used this technique with measurements in specific wavelength bands to determine smoke concentrations.
One major drawback of Schmidt type methods is that they give non-linearly weighted averages of unknown quantities across the width of the combustor, and local variations along the line of sight cannot be deduced. Moreover, on a test rig, there may not be two appropriately aligned holes in the combustor walls for the source and detector, so modification of the combustor may be required; and on a working gas turbine engine aligned holes are out of the question. Furthermore, the local gas composition cannot be deduced. Present gas analysis techniques involve taking gas samples, which is difficult to arrange without disturbing the combustion process.
It has also been proposed to measure temperature and soot concentration by traversing across a flame (Beer and Claus, "The traversing method of radiation measurement in luminous flames", J. Inst. Fuel, vol. 35 pp. 437-443, 1962). A development of this technique in which spectral measurements (i.e., measurements of radiation at a plurality of wavelengths) are made is described by Hammond and Beer, "Spatial Distribution of Spectral Radiant Energy in a Pressure Jet Oil Flame", Heat Transfer in Flames (Chapter 19), Scripta Book Co., 1974. In this technique, a sight tube from the source furnace is traversed across the flame, and averaged measurements are obtained along a line of sight from the position of the end of the sight tube in the flame to the detector. Thus, this technique still has the problems noted above of requiring a line of sight and of providing only averaged measurements (from the position of the end of the sight tube in the flame to the external detector). There is no correlation between measurements at different positions along the traverse of the sight tube.